Method for forming nano-scale metal particles

ABSTRACT

A method for forming nano-scale metal particles by a novel reducing agent is described. The method can be carried out at room temperature and under an atmospheric environment by relatively simple processes to prepare nano-scale metal particles with a diameter less than 20 nm. This method comprises the following steps. At first, a first blending process is performed to blend a metal salt and a first solvent together to form a first solution. Then, a second blending process is performed to blend a reducing agent and a second solvent together to form a second solution. The reducing agent comprises one compound selected from the group consisting of the following or combination thereof: boron-containing hydride and boron-containing hydrocarbon. Following that, a third blending process is performed to blend the first solution and the second solution together to form a third solution. Finally, the reducing agent is used to reduce the metal salt in the third solution to form the nano-scale metal particles. In addition, if a dispersing agent is added after the nano-scale metal particles are formed, the nano-scale metal particles can have a particle diameter less than 10 nm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to a method for formingnano-scale particles and more particularly to a method for formingnano-scale particles by a novel reducing agent.

2. Description of the Prior Art

A nano-scale metal material means a material containing nano-scale metalparticles or having the nano-scale structure in the matrix thereof. Asthe diameter of metal particles is within the nano scale, the surfacearea of particles becomes very large and new electrical, magnetic,optical, and chemical characteristics different from the bulk materialthereof are appeared due to the particle diameter being less than thelight wavelength so that these nano-scale particles can be applied invarious fields, such as electrode materials, conducting films,biochemical sensing, drug delivery, optical sensing, catalyzed reaction,and electrical engineering.

Nano-scale metal materials can be categorized into nano-scale metalparticles, nano-wires, nano-membranes, nano bulk materials. The laterthree forms can be derived from the first one, that is, nano-scaleparticles. Therefore, the preparation and development of nano-scalemetal particles are more important than that of the rest forms ofnano-scale metal materials. A method for forming nano-scale metalparticles to effectively control the particle diameter, the distributionof the particle diameters, particle types, and crystal structures, etc.,is the current research target.

Currently, the chemical reduction method is commonly used to preparenano-scale metal particles. The chemical reduction method uses areducing agent or an electrochemical system to reduce metal oxide intometal in a free space or confined space.

In the above chemical reduction method, the reducing agent in use caneasily react with oxygen or moisture to result in burning or anexplosion. Base on safety consideration, it should take place under aninactive environment (without oxygen). By doing so, the production costwill be increased.

In light of the above description, a method under mild reactionconditions, such as at room temperature and under an atmosphericenvironment, to form nano-scale metal particles with shorter reactiontime is an important technical development topic for the industry.

SUMMARY OF THE INVENTION

In light of the above background, in order to fulfill the industrialrequirements, the invention provides a method for forming nano-scalemetal particles.

The invention discloses a method for forming nano-scale metal particlesby a novel reducing agent. The method can be carried out at roomtemperature and under an atmospheric environment by relatively simpleprocesses to prepare nano-scale metal particles with a diameter lessthan 20 nm. This method comprises the following steps. At first, a firstblending process is performed to blend a metal salt and a first solventtogether to form a first solution. Then, a second blending process isperformed to blend a reducing agent and a second solvent together toform a second solution. The reducing agent comprises one compoundselected from the group consisting of the following or combinationthereof: boron-containing hydride and boron-containing hydrocarbon.Following that, a third blending process is performed to blend the firstsolution and the second solution together to form a third solution.Finally, the reducing agent is used to reduce the metal salt in thethird solution to form the nano-scale metal particles. In addition, if adispersing agent is added after the nano-scale metal particles areformed, the nano-scale metal particles can have a particle diameter lessthan 10 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are particle size spectra of tin-containingnano-scale metal particles;

FIG. 2 shows TEM images of tin-containing nano-scale metal particles;

FIG. 3A and FIG. 3B show TEM images of copper nano-scale metalparticles. Image analysis of 30 nm to 60 nm; and

FIG. 4A and FIG. 4B show TEM images of nano-scale metal particles. Imageanalysis of 30 nm to 60 nm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first embodiment of the invention, a method for forming nano-scalemetal particles at room temperature is disclosed. At first, a firstblending process is performed to blend a metal salt and a first solventtogether to form a first solution. Then, a second blending process isperformed to blend a reducing agent and a second solvent together toform a second solution. The reducing agent comprises one compoundselected from the group consisting of the following or combinationthereof: boron-containing hydride and boron-containing hydrocarbon.Finally, a third blending process is performed to blend the firstsolution and the second solution together to form a third solution. Thereducing agent is used to reduce the metal salt in the third solution toform the nano-scale metal particles.

The temperature of the first, second, and third blending processes isless than or equal to 40° C. In addition, the processes are carried outunder an atmospheric environment. Preferably, the processes are carriedout under an inactive environment.

Moreover, in the first solution, the molar concentration of the metalsalt is less than or equal to 10⁻⁴M while in the second solution themolar concentration of the reducing agent is less than or equal to10⁻⁴M.

Besides, the reducing agent is tetraethylammonium borohydride. The metalsalt has a general formula: MX where M is selected from the groupconsisting of the following: tin, copper, silver, and gold; and X isselected from the group consisting of the following: halogen, sulfateion, phosphate ion, sulfonate ion, nitrate ion, and carboxylate ion.

In a preferred example of this embodiment, the nano-scale metalparticles are dispersed in solution by a dispersing agent after formedwhere the dispersing agent is selected from the group consisting of thefollowing: water, alcohol, n-hexane, toluene, and tetrahydrofuran. Inaddition, the first solvent and the second solvent are independentlyselected from the group consisting of the following or combinationthereof: water, alcohol, and a polar solvent. On the other hand, themolar ratio of the reducing agent to the metal salt is 60˜300. Theparticle diameter of the formed nano-scale metal particles is 5˜70 nmand preferably less than 10 nm.

In another preferred example of this embodiment, no additional additiveor dispersing agent is needed after the nano-scale metal particles areformed. Furthermore, the first solvent and the second solvent areindependently selected from the group consisting of the following:N,N-dimethyl-acetamide (DMAC), dimethyl-sulfoxide (DMSO), and1-methyl-2-pyrrolidinone (NMP). On the other hand, the molar ratio ofthe reducing agent to the metal salt is 5˜25. The particle diameter ofthe formed nano-scale metal particles is 15˜60 nm and preferably lessthan 20 nm.

In a second embodiment of the invention, a method for formingtin-containing nano-scale metal particles at room temperature isdisclosed. At first, a blending process is performed to blend a tinsalt, a reducing agent, and a solvent together to form a mixturesolution. The reducing agent is used to reduce the metal salt in themixture solution to form the tin-containing nano-scale metal particles.The mixture solution selectively comprises other metal salts and thereducing agent comprises one compound selected from the group consistingof the following or combination thereof: boron-containing hydride andboron-containing hydrocarbon.

The blending process is carried out under an atmospheric environment.The temperature of the blending processes is less than or equal to 40°C. In addition, the nano-scale metal particles are dispersed in solutionby a dispersing agent after formed where the dispersing agent isselected from the group consisting of the following: water, alcohol,n-hexane, toluene, and tetrahydrofuran.

The reducing agent is tetraethylammonium borohydride. The solvent isselected from the group consisting of the following or combinationthereof: water, alcohol, and a polar solvent. The tin salt has a generalformula: SnX where X is selected from the group consisting of thefollowing: halogen, sulfate ion, phosphate ion, sulfonate ion, nitrateion, and carboxylate ion. On the other hand, the other metal salt isselected from the group consisting of the following or combinationthereof: silver salt, copper salt, and gold salt. In addition, the molarratio of the reducing agent to all of the metal salts is 60˜300. Theparticle diameter of the tin-containing nano-scale metal particlesformed in this embodiment is 5˜70 nm and preferably less than 10 nm.

In a third embodiment of the invention, a method for forming coppernano-scale metal particles is disclosed. At first, a first blendingprocess is performed to blend a copper salt and a first solvent togetherto form a first solution. Then, a second blending process is performedto blend a reducing agent and a second solvent together to form a secondsolution. The reducing agent comprises one compound selected from thegroup consisting of the following or combination thereof:boron-containing hydride and boron-containing hydrocarbon. The secondsolvent is independently selected from the group consisting of thefollowing: N,N-dimethyl-acetamide (DMAC), dimethyl-sulfoxide (DMSO), and1-methyl-2-pyrrolidinone (NMP). Finally, a third blending process isperformed to blend the first solution and the second solution togetherto form a third solution. The reducing agent is used to reduce thecopper salt in the third solution to form dispersed copper nano-scalemetal particles.

The first, second, and third blending processes are carried out under anitrogen environment. The temperature of the first, second, and thirdblending processes is less than or equal to 40° C. Moreover, in thefirst solution, the molar concentration of the metal salt is preferablyless than or equal to 10⁻⁴M while in the second solution the molarconcentration of the reducing agent is preferably less than or equal to10⁻⁴M. In addition, the molar ratio of the reducing agent to thecopper-containing metal salts is 5˜25.

The reducing agent is tetraethylammonium borohydride. The copper salthas a general formula: CuX where X is selected from the group consistingof the following: halogen, sulfate ion, phosphate ion, sulfonate ion,nitrate ion, and carboxylate ion.

On the other hand, the first solvent is independently selected from thegroup consisting of the following or combination thereof:N,N-dimethyl-acetamide (DMAC), dimethyl-sulfoxide (DMSO), and1-methyl-2-pyrrolidinone (NMP). In a preferred example of thisembodiment, the first solvent is N,N-dimethyl-acetamide (DMAC) while thesecond solvent is dimethyl-sulfoxide (DMSO).

The particle diameter of the copper nano-scale metal particles formed inthis embodiment is 15˜60 nm and preferably less than 20 nm.

EXAMPLE 1 Formation and Properties of Tin-Containing Nano-Scale MetalParticles

is example is to prepare and investigate the tin-containing nano-scalemetal particles according to the invention. The chemical equation isshown as the following:

SnCl₂+2N(Et)₄(BH₄)→Sn+2N(Et)₄Cl+B₂H₆+H₂.

e detailed steps are given in the following. At room temperature andunder an atmospheric environment, a certain amount of the reducing agentand a certain amount of SnCl₂ are weighted and placed in a 50 mlgraduate cylinder, separately. A magnet is placed in the graduatecylinder for stirring beforehand. Then, a septum is used to seal thecontainer and the septum is then wrapped with paraffin for air-tight.Nitrogen gas is introduced into the graduate cylinder to expel themoisture in air. 20 ml of solvent is added by a syringe and then themixture is stirred for 30 minutes to ensure completely dissolving in thesolvent. Thus, the reducing agent solution and the SnCl₂ solution areprepared. The reducing agent solution and the SnCl₂ solution withdifferent quantities are taken and mixed under a nitrogen environment.After being stirred, the mixture solution is tested by instruments.

this example, water, alcohols (such as methanol, ethanol, butanol,ethylene glycol), and polar solvents (such as DMAC and NMP) are used asthe solvent. It is found that the samples using DMAC and NMP as thesolvent have better results and the particle diameter can be controlledeasily as well. Besides, according the test results, the added quantityof the SnCl₂ solution has great influence on the particle diameter. If asmall quantity of the reducing agent solution is added into a largequantity of the SnCl₂ solution, the particle diameter of the obtainedparticles is relatively large after analyzed and can not be nano-scale.If a small quantity of the SnCl₂ solution is added into a large quantityof the reducing agent solution, the particle diameter of the obtainedparticles is clearly relatively small.

According to the above results, this example uses DMAC as the solvent toform tin-containing nano-scale metal particles. As shown in FIGS. 1A and1B, when less than 1000 μl of tin chloride is added, the particlediameter of the formed tin nano-scale metal particles is less than 50nm. When 400 μl and 600 μl of tin chloride are added, the averageparticle diameters are 16 nm and 34 nm, respectively. Since the datashow the average particle diameter, the particle diameter being lessthan 10 nm can also be seen in the figure. This example also tests thesame sample by transmission electron microscopy (TEM). The sample isdripped on copper gauze coated with carbon film. The excess liquid isremoved and the sample is dried and ready for investigation. The resultis shown in FIG. 2.

EXAMPLE 2 Formation of Copper Nano-Scale Metal Particles

The chemical equation for forming copper nano-scale metal particlesaccording to the invention is shown as the following:

CuCl₂+2N(C₂H₅)₄BH₄→Cu+2N(C₂H₅)₄Cl+B₂H₆+H₂.

The detailed steps are given in the following. At room temperature andunder a nitrogen environment, a proper quantity of the reducing agent isweighted. The reducing agent is tetraethylammonium borohydride. Thereducing agent dissolves in the solvent to form 20 ml of 0.01M reducingagent solution. Then, at room temperature and under a nitrogenenvironment, a proper quantity of copper chloride is weighted anddissolves in the solvent to form 20 ml of 0.005M copper chloridesolution. These solutions are separately stirred by magnets for over 20minutes to ensure completely dissolution. Finally, under a nitrogenenvironment, the reducing agent solution is blended with the copperchloride solution with different ratios. An ultrasonic vibrator is usedwhile the reaction takes place for 15 minutes. Thus, the solutioncontaining copper nano-scale metal particles is obtained.

N,N-dimethyl-acetamide (DMAC) is used as the solvent for the reducingagent and the metal salt, the copper nano-scale metal particles with theparticle diameter of 30˜60 nm can be formed. The TEM pictures of thecopper nano-scale metal particles are shown in FIGS. 3A and 3B. Themagnification ratio in FIG. 3A is 50,000 while the magnification ratioin FIG. 3B is 100,000.

N,N-dimethyl-acetamide (DMAC) is used as the solvent for copper chlorideand dimethyl-sulfoxide (DMSO) is used as the solvent for the reducingagent, the copper nano-scale metal particles with the particle diameterof 15˜30 nm can be formed. The TEM pictures of the copper nano-scalemetal particles are shown in FIGS. 4A and 4B. The magnification ratio inFIGS. 4A and 4B is 100,000.

Obviously many modifications and variations are possible in light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims the present invention can be practiced otherwisethan as specifically described herein. Although specific embodimentshave been illustrated and described herein, it is obvious to thoseskilled in the art that many modifications of the present invention maybe made without departing from what is intended to be limited solely bythe appended claims.

1. A method for forming nano-scale metal particles, comprising:performing a first blending process to blend a metal salt and a firstsolvent together to form a first solution; performing a second blendingprocess to blend a reducing agent and a second solvent together to forma second solution wherein said reducing agent comprises one compoundselected from the group consisting of the following or combinationthereof: boron-containing hydride and boron-containing hydrocarbon;performing a third blending process to blend said first solution andsaid second solution together to form a third solution; and using saidreducing agent to reduce said metal salt in said third solution to formsaid nano-scale metal particles.
 2. The method according to claim 1,wherein the temperature in said first, second, and third blendingprocesses is less than or equal to 40° C.
 3. The method according toclaim 1, wherein in said first solution the molar concentration of saidmetal salt is less than or equal to 10⁻⁴M.
 4. The method according toclaim 1, wherein in said second solution the molar concentration of saidreducing agent is less than or equal to 10⁻⁴M.
 5. The method accordingto claim 1, wherein said reducing agent is tetraethylammoniumborohydride.
 6. The method according to claim 1, wherein said metal salthas a general formula: MX where M is selected from the group consistingof the following: tin, copper, silver, and gold; and X is selected fromthe group consisting of the following: halogen, sulfate ion, phosphateion, sulfonate ion, nitrate ion, and carboxylate ion.
 7. The methodaccording to claim 1, wherein said first solvent and said second solventare independently selected from the group consisting of the following orcombination thereof: water, alcohol, and a polar solvent.
 8. The methodaccording to claim 7, wherein said nano-scale metal particles aredispersed in solution by a dispersing agent after formed where saiddispersing agent is selected from the group consisting of the following:water, alcohol, n-hexane, toluene, and tetrahydrofuran.
 9. The methodaccording to claim 7, wherein the molar ratio of said reducing agent tosaid metal salt is between 60 and
 300. 10. The method according to claim7, wherein the particle diameter of said formed nano-scale metalparticles is 5˜70 nm.
 11. The method according to claim 1, wherein saidfirst solvent and said second solvent are independently selected fromthe group consisting of the following or combination thereof:N,N-dimethyl-acetamide (DMAC), dimethyl-sulfoxide (DMSO), and1-methyl-2-pyrrolidinone (NMP).
 12. The method according to claim 11,wherein the molar ratio of said reducing agent to said metal salt isbetween 5 and
 25. 13. The method according to claim 11, wherein theparticle diameter of said formed nano-scale metal particles is 15˜60 nm.14. A method for forming tin-containing nano-scale metal particles,comprising: performing a blending process to blend a tin salt, areducing agent, and a solvent together to form a mixture solutionwherein said mixture solution selectively comprises other metal saltsand said reducing agent comprises one compound selected from the groupconsisting of the following or combination thereof: boron-containinghydride and boron-containing hydrocarbon; and using said reducing agentto reduce said metal salt in said mixture solution to form saidtin-containing nano-scale metal particles.
 15. The method according toclaim 14, wherein the temperature in said blending processes is lessthan or equal to 40° C.
 16. The method according to claim 14, whereinsaid blending process is performed under an atmospheric environment. 17.The method according to claim 14, wherein said reducing agent istetraethylammonium borohydride.
 18. The method according to claim 14,wherein said solvent is selected from the group consisting of thefollowing or combination thereof: water, alcohol, a polar solvent. 19.The method according to claim 14, wherein said tin salt has a generalformula: SnX where and X is selected from the group consisting of thefollowing: halogen, sulfate ion, phosphate ion, sulfonate ion, nitrateion, and carboxylate ion.
 20. The method according to claim 14, whereinsaid other metal salt is selected from the group consisting of thefollowing or combination thereof: silver salt, copper salt, and goldsalt.
 21. The method according to claim 14, wherein said nano-scalemetal particles are dispersed in solution by a dispersing agent afterformed where said dispersing agent is selected from the group consistingof the following: water, alcohol, n-hexane, toluene, andtetrahydrofuran.
 22. The method according to claim 14, wherein the molarratio of said reducing agent to all of said metal salts is between 60and
 300. 23. The method according to claim 14, wherein the particlediameter of said formed tin-containing nano-scale metal particles is5˜70 nm.
 24. A method for forming copper nano-scale metal particles,comprising: performing a first blending process to blend a copper saltand a first solvent together to form a first solution; performing asecond blending process to blend a reducing agent and a second solventtogether to form a second solution wherein said reducing agent comprisesone compound selected from the group consisting of the following orcombination thereof: boron-containing hydride and boron-containinghydrocarbon and said second solvent is selected from the groupconsisting of the following: N,N-dimethyl-acetamide (DMAC),dimethyl-sulfoxide (DMSO), and 1-methyl-2-pyrrolidinone (NMP);performing a third blending process to blend said first solution andsaid second solution together to form a third solution and using saidreducing agent to reduce said copper salt in said third solution to formdispersed copper nano-scale metal particles.
 25. The method according toclaim 24, wherein the temperature of said first, second, and thirdblending processes is less than or equal to 40° C.
 26. The methodaccording to claim 24, wherein said first, and second, and thirdblending processes are performed under a nitrogen environment.
 27. Themethod according to claim 24, wherein in said first solution the molarconcentration of said metal salt is less than or equal to 10⁻⁴M.
 28. Themethod according to claim 24, wherein in said second solution the molarconcentration of said reducing agent is less than or equal to 10⁻⁴M. 29.The method according to claim 24, wherein said reducing agent istetraethylammonium borohydride.
 30. The method according to claim 24,wherein said first solvent is independently selected from the groupconsisting of the following or combination thereof:N,N-dimethyl-acetamide (DMAC), dimethyl-sulfoxide (DMSO), and1-methyl-2-pyrrolidinone (NMP).
 31. The method according to claim 24,wherein said first solvent is N,N-dimethyl-acetamide (DMAC) and saidsecond solvent is dimethyl-sulfoxide (DMSO).
 32. The method according toclaim 24, wherein said copper salt has a general formula: CuX where andX is selected from the group consisting of the following: halogen,sulfate ion, phosphate ion, sulfonate ion, nitrate ion, and carboxylateion.
 33. The method according to claim 24, wherein the molar ratio ofsaid reducing agent to said copper-containing metal salts is between 5and
 25. 34. The method according to claim 14, wherein the particlediameter of said formed copper nano-scale metal particles is 15˜60 nm.